As motor-driving power sources for vehicles such as electric automobiles, high-performance secondary batteries have been recently developed. For secondary batteries to be used for motor driving, high capacities and excellent cycle characteristics are required. Accordingly, active development is being promoted of lithium ion secondary batteries with high theoretical energies.
As the negative electrode materials of existing lithium ion secondary batteries, there are known carbon-based materials, graphite-based materials, oxide-based materials such as CoO, Co3O4, Fe2O3, metal nitride-based materials such as Ge3N4, Zn3N2, and Cu3N, Li—Si-M-based materials such as Mg2Si, CrSi2, and NiSi, a Li metal, and a Li alloy. In practice, however, carbon-based materials and graphite-based materials are mainly used. In addition, there is known a non-aqueous electrolyte secondary battery using as a negative electrode material a metal carbide such as Cr4C, VC2, Fe2C, or FeC (refer to Patent Literature 1), although such a non-aqueous electrolyte secondary battery is difficult to have a high capacity since the discharge capacity of its test cell is approximately 500 mAh/g, compared to a discharge capacity of 350 mAh/g obtained in a case where a graphite-based material is used for a negative electrode in a comparative example.
As materials that can offer higher capacities and higher energy densities to replace intercalation materials including carbon-based materials and graphite-based materials, Sn and Si alloyed with Li, and materials based on alloys of Sn and Si are receiving attention as negative electrode materials (refer to Non-Patent Literature 1).
Further, unlike the intercalation materials, an iron oxide such as Fe2O3 performs, as a negative electrode active material, charge and discharge reactions of a conversion type (decomposition and reproduction type). For example, it is reported that as a formula Fe2O3+6Li→3Li2O+2Fe shows, Fe2O3 decomposes and reduces when it absorbs Li ions at the time of charging so that iron (Fe) and lithium oxide (Li2O) are generated and that iron oxide (Fe2O3) is reproduced when the Li ions are desorbed at the time of discharging. Patents have been applied for a lithium secondary battery (refer to Patent Literature 2) having a negative electrode that includes an iron oxide film on a rough-surfaced conductive substrate as the conversion-type negative electrode active material, and further for a lithium secondary battery (refer to Patent Literature 3) using as the conversion-type negative electrode active material powders of iron oxide with particle diameters of 1 μm to 20 μm and crystallite sizes of 600 Å or smaller.
A negative electrode active material is usually used by being applied on a negative electrode collector in a mixture with a conduction assistant or a binder. For the collector, aluminum, titanium, copper, iron, stainless steel, etc. are used. Another patent (refer to Patent Literature 4) has been applied for a lithium battery that uses lithium foil or lithium alloy foil as a negative electrode active material. The lithium battery according to Patent Literature 4 is characterized in that the lithium foil or the lithium alloy foil is in a direct contact with a metal collector board of a stainless steel, for example, and that the main surface of the board is a rough surface provided with crater-like spots, by laser processing, having diameters of approximately 20 μm to 100 μm and differences in height of approximately 0.5 μm to 5 μm. The rough surfacing is performed for the purpose of enhancing the adhesive properties between the collector and the lithium foil.
Moreover, laser marking is carried out as processing of inscribing letters, figures, patterns, etc. on various materials. As an example of the laser marking, there is known a surface processing method with which a surface of Ti, a stainless steel, etc. is irradiated with a YVO4 laser having a spot diameter of 20 μm to 80 μm, whereby an ornament excellent in durability and aesthetically pleasing can be created (refer to Patent Literature 5). This method, however, is not used for forming a surface layer part having a functionality of a chemical reaction, for example.